1. Field of the Invention
The present invention generally relates to a micro oscillating element having an oscillation section capable of rotary displacement. In particular, the present invention relates to a micromirror element, an acceleration sensor, an angular velocity sensor, and a vibration element, for example.
2. Description of the Related Art
In recent years, elements having a microstructure formed by micromachining technology have been put to practical use in various technological fields. In the field of optical communication technology, for example, minute micromirror elements having a light reflecting function are gaining attention.
In optical communication, optical signals are transmitted using optical fiber as a medium, and an optical switching device is typically used to switch the transmission path of the optical signal from one fiber to another fiber. To achieve favorable optical communication, the characteristics required of the optical switching device include a large capacity, high velocity, and high reliability during the switching operation. With these considerations in mind, high expectations are being placed on optical switching devices incorporating a micromirror element manufactured using micromachining technology. A micromirror element is capable of performing switching processing between an input side optical transmission path and an output side optical transmission path in the optical switching device without converting the optical signal into an electrical signal, and is therefore suitable for obtaining the characteristics listed above.
A micromirror element comprises a mirror surface for reflecting light, and is capable of varying the direction in which the light is reflected by oscillating the mirror surface. Electrostatic micromirror elements which use electrostatic force to tilt the mirror surface are employed in many devices. Electrostatic micromirror elements can be divided into two main types, those manufactured by so-called surface micromachining technology, and those manufactured by so-called bulk micromachining technology.
In surface micromachining technology, material thin film corresponding to each constitutional region is machined into a desired pattern, and such patterns are laminated successively to form the various regions constituting the element, such as a supporting and fixing portion, an oscillation section, a mirror surface, and an electrode portion, and a sacrificial layer which is removed at a later stage. In bulk micromachining technology, on the other hand, a material substrate is itself etched to form the fixing and supporting portion, the oscillation section, and so on into a desired form, whereupon the mirror surface and electrodes are formed with thin film. Bulk micromachining technology is described in Japanese Unexamined Patent Application Publication H9-146032, Japanese Unexamined Patent Application Publication H9-146034, Japanese Unexamined Patent Application Publication H10-190007, and Japanese Unexamined Patent Application Publication 2000-31502, for example.
One of the technological items required of a micromirror element is that the mirror surface for reflecting light has a high degree of flatness. However, with surface micromachining technology, the mirror surface that is ultimately formed is thin, and therefore buckles easily. Accordingly, it is difficult to achieve a high degree of flatness on a mirror surface with a large surface area. Conversely, with bulk micromachining technology, the relatively thick material substrate itself is cut into by etching technology to form a mirror supporting portion, and the mirror surface is provided on the mirror supporting portion. Hence the rigidity of even a mirror surface with a large surface area can be secured. As a result, a mirror surface having a sufficiently high degree of optical flatness can be formed.
FIG. 32 is a partial perspective view of a conventional micromirror element X6 manufactured according to bulk micromachining technology. The micromirror element X6 comprises a mirror supporting portion 61 provided with a mirror surface 64 on its upper face, a frame 62 (partially omitted from the drawing), and a pair of torsion bars 63 connecting the mirror supporting portion 61 and frame 62. Comb-tooth electrodes 61a, 61b are formed on the pair of end portions of the mirror supporting portion 61. A pair of inwardly-extending comb-tooth electrodes 62a, 62b is formed on the frame 62 corresponding to the comb-tooth electrodes 61a, 61b. The pair of torsion bars 63 defines an oscillation axis A6 of the oscillating operation of the mirror supporting portion 61 in relation to the frame 62.
In the micromirror element X6 constituted in this manner, one set of the comb-tooth electrodes provided close to each other for generating a driving force (electrostatic attraction), for example the comb-tooth electrodes 61a and 62a, are oriented in two tiers when no voltage is applied, as shown in FIG. 33A. However, when a predetermined voltage is applied, the comb-tooth electrode 61a is attracted toward the comb-tooth electrode 62a, as shown in FIG. 33B, whereby the mirror supporting portion 61 is rotationally displaced. More specifically, when the comb-tooth electrode 61a is charged positively and the comb-tooth electrode 62a is charged negatively, the comb-tooth electrode 61a is attracted toward the comb-tooth electrode 62a, and thereby the mirror supporting portion 61 is rotationally displaced about the oscillation axis A6 with the torsion bars 63 being twisted. By driving the mirror supporting portion 61 to tilt in this manner, the reflection direction of the light that is reflected by the mirror surface 64 provided on the mirror supporting portion 61 is switched.
To miniaturize the micromirror element X6 along the oscillation axis A6, the length L61 of the mirror supporting portion 61, which occupies most part of the element, need be shortened. However, shrinking the length L61 cannot easily be compatible with maintaining the driving force enough to oscillate the mirror supporting portion 61.
In the micromirror element X6, the plurality of electrode teeth of the respective comb-tooth electrodes 61a, 61b are supported on the mirror supporting portion 61 at intervals in the oscillation axis A6 direction, and therefore the number of electrode teeth of the comb-tooth electrodes 61a, 61b is restricted by the length L61 of the mirror supporting portion 61. As a result, the number of electrode teeth constituting the set of comb-tooth electrodes 61a, 62a and the number of electrode teeth constituting the set of comb-tooth electrodes 61b, 62b are restricted by the length L61 of the mirror supporting portion 61. Furthermore, in order to secure enough driving force to drive the oscillating operation of the mirror supporting portion 61, or in other words to secure the electrostatic attraction that can be generated between the comb-tooth electrodes 61a, 62a and the comb-tooth electrodes 61b, 62b, a sufficient surface area to allow the electrode teeth of the set of comb-tooth electrodes 61a, 62a to face each other and a sufficient surface area to allow the electrode teeth of the set of comb-tooth electrodes 61b, 62b to face each other must be secured. To secure such a surface area enabling the electrode teeth to face each other when the length L61 of the mirror supporting portion 61 has been reduced, a method of reducing a width d1 of each electrode tooth and narrowing a gap d2 between the electrode teeth such that the number of electrode teeth of the comb-tooth electrodes 61a, 61b, 62a, 62b is set at no less than a fixed number, or a method of increasing the distance between the mirror supporting portion 61 and the frame 62 and increasing a length d3 of each electrode tooth, may be considered.
However, reducing the width d1 and increasing the length d3 of the electrode teeth lead to a reduction in mechanical strength in the width direction of the electrode teeth. As a result, when a voltage is applied as described above with reference to FIG. 33B, the electrode teeth deform in the width direction thereof, causing a defect whereby the electrode teeth stick to adjacent teeth. Further, reducing the gap d2 between the electrode teeth leads to difficulties in the manufacturing process of the micromirror element X6, decreases in yield, and so on.
Hence there are difficulties involved in miniaturizing the micromirror element X6 through contraction of the oscillation axis A6 direction while maintaining enough driving force to drive the oscillating operation of the mirror supporting portion 61. In micro oscillating elements such as the micromirror element X6, a characteristic whereby large rotary displacement and a high speed oscillating operation can be realized at a low drive voltage is typically demanded of the region in which the oscillating operation takes place, but in order to obtain such a characteristic, the driving force for driving the oscillating operation of the oscillation section must be held at no less than a fixed level.